Magnetoresistive effect element and magnetic memory

ABSTRACT

A magnetoresistive effect element includes a nonmagnetic layer having mutually facing first and second surfaces. A reference layer is provided on the first surface and has a fixed magnetization direction. A magnetization variable layer is provided on the second surface, has variable magnetization direction, and has a planer shape including a rectangular part, a first projected part, and a second projected part. The rectangular part has mutually facing first and second longer sides and mutually facing first and second shorter sides. The first projected part projects from the first longer side at a position shifted from the center toward the first shorter side. The second projected part projects from the second longer side at a position shifted from the center toward the second shorter side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2005-207499, filed Jul. 15, 2005,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetoresistive effect element, andto a magnetic memory. For example, the present invention relates to theshape of a ferromagnetic tunnel junction element.

2. Description of the Related Art

There has been a magnetic memory using a ferromagnetic tunnel junction(MTJ) element as a memory cell. The MTJ element is mainly composed offerromagnetic layer/insulating layer/ferromagnetic layer, which aresuccessively stacked. A junction resistance to current tunneling theinsulating layer to flow is minimum when the magnetization directions oftwo ferromagnetic layers are parallel to each other and is maximum whenthe magnetization directions of two ferromagnetic layers areanti-parallel to ach other. This is called tunnel magnetoresistiveeffect.

One ferromagnetic layer is called a reference layer whose magnetizationdirection is fixed. The other ferromagnetic layer is called a recordinglayer whose magnetization direction changes. Parallel or anti-parallelstate of the magnetization direction of the reference and storage layersis made to correspond to binary information, and thereby, information isstored. Write current flowing a write line that is provided around amemory cell generates a magnetic field to switch the magnetization ofthe recording layer to write information.

An advance in the scale-down of memory cell in order to improve theintegration of magnetic memories reduces a ferromagnetic materialforming the memory cell. Smaller ferromagnetic material generally haslarger coercivity. Larger coercivity requires greater switching magneticfiled for switching the magnetization direction of a recording layer.Thus, a write current required for generating a switching magnetic filedbecomes large; as a result, power consumption is increased. Therefore,in order to realize lower power consumption, it is important to reducethe coercivity of the small magnetic material.

Meanwhile, a parameter called a thermal fluctuation constant exists asan index of memory cell stably holding information for long term. Thethermal fluctuation appears to be proportional to volume and coercivityin general. Therefore, high thermal stability is required to holdinformation for long term; nevertheless, reduced power consumptiondegrades the thermal stability.

Moreover, an edge domain gives an influence to a switching magneticfield. The edge domain is a special magnetic domain and appears aroundan edge of a rectangular micro ferromagnetic material whose width of theshorter axis is, for example, less than sub-micron from several microns.The edge domain is disclosed in the document, J. App. Phys. 81, 5471(1997)(J. App. Phys. 81, 5471 (1997)), for example.

The edge domain is generated because the magnetization is given alongthe shorter side of rectangle and form a spirally rotating pattern inorder to reduce anti-magnetic energy in the shorter side. Themagnetization is generated in the direction different from the middleportion in both edge portions of the magnetic material although it isgenerated in the direction according to magnetic anisotropy in themiddle portion of the magnetic material. During spin switching in therectangular ferromagnetic material, the edge domain is known to grow itssize. Thus, the edge domain serves to increase a switching magneticfiled.

The edge domain is sensitive to the shape of the ferromagnetic material.For this reason, the use of an elliptic ferromagnetic material isproposed in U.S. Pat. No. 5,757,695.

Moreover, U.S. Pat. No. 5,748,524 and JPN. PAT. APPLN. KOKAI PublicationNo. 2000-100153 disclose that the edge domain is fixed to prevent acomplicate change of magnetic structure from occurring in spin switchingas much as possible. The technique can control magnetization behavior inspin switching but cannot reduce a switching magnetic filed. Moreover,another structure must be added to fix the edge domain; therefore, thisis not suitable for achieving high density.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda magnetoresistive effect element comprising: a nonmagnetic layer havinga first surface and a second surface which face each other; a referencelayer provided on the first surface and having a fixed magnetizationdirection; and a magnetization variable layer provided on the secondsurface, having variable magnetization direction, and having a planershape including a rectangular part, a first projected part, and a secondprojected part, the rectangular part having a first longer side and asecond longer side which face each other and a first shorter side and asecond shorter side which face each other, the first projected partprojecting from the first longer side at a position shifted from thecenter toward the first shorter side, the second projected partprojecting from the second longer side at a position shifted from thecenter toward the second shorter side.

According to a second aspect of the present invention, there is provideda magnetoresistive effect element comprising: a first nonmagnetic layerhaving a first surface and a second surface which face each other; afirst reference layer provided on the first surface and having a fixedmagnetization direction; a second nonmagnetic layer having a thirdsurface and a fourth surface which face each other; a second referencelayer provided on the third surface and having a fixed magnetizationdirection; and a magnetization variable layer interposed between thesecond and fourth surfaces, having variable magnetization direction, andhaving a planer shape including a rectangular part, a first projectedpart, and a second projected part, the rectangular part having a firstlonger side and a second longer side which face each other and a firstshorter side and a second shorter side which face each other, the firstprojected part projecting from the first longer side at a positionshifted from the center toward the first shorter side, the secondprojected part projecting from the second longer side at a positionshifted from the center toward the second shorter side.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a top plan view showing a MTJ element according to oneembodiment of the present invention;

FIG. 2 and FIG. 3 are perspective views showing a MTJ element accordingto one embodiment of the present invention;

FIG. 4 is a view showing the state of magnetization when a magneticfield is applied to the MTJ element shown in FIG. 1 in one directiononly;

FIG. 5 is a graph showing a magnetization curve obtained by simulationrelevant to the MTJ element of FIG. 1;

FIG. 6 is a graph showing an asteroid curve obtained by simulationrelevant to the MTJ element of FIG. 1;

FIG. 7 is a top plan view showing another MTJ element according to oneembodiment of the present invention;

FIG. 8 is a view showing the state of magnetization when a magneticfield is applied to another MTJ element shown in FIG. 7 in one directiononly;

FIG. 9 is a graph showing a magnetization curve obtained by simulationrelevant to another MTJ element of FIG. 7;

FIG. 10 is a view showing one example of a planar layout of a cell arrayof a MRAM;

FIG. 11 is a view showing a memory cell having a cross point structure;and

FIG. 12 is a view showing a memory cell having a 1Tr+1MTJ structure.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention will be described below withreference to the accompanying drawings. In the following description,the same reference numerals are used to designate components having theidentical function and configuration, and the overlapping explanation ismade if necessary.

One embodiment of the present invention will be explained using oneexample of computer simulation results. FIG. 1 is a top plan viewshowing a MTJ element (magnetoresistive effect element, magneticrecording element) according to one embodiment of the present invention.As shown in FIG. 2, an MTJ element 1 has at least ferromagnetic layer11, insulating layer 12 and ferromagnetic layer 13, which aresuccessively stacked.

The ferromagnetic layers 11 and/or 13 may have a stacked structurecomprising several sub-layers. The magnetization direction of oneferromagnetic layer (e.g., ferromagnetic layer 11) is fixed. This isachieved by providing an anti-ferromagnetic layer 14 below theferromagnetic layer 11. The ferromagnetic layer 11 is hereinafterreferred to as a reference layer.

On the other hand, the fixing mechanism is not given with respect to themagnetization direction of the ferromagnetic layer 13. Thus, themagnetization direction of the ferromagnetic layer 13 is variable. Theferromagnetic layer 13 is hereinafter referred to as a magnetizationvariable layer (recording layer). Ferromagnetic layers 11, 13 andinsulating layer 12 have a planar shape shown in FIG. 1.

As seen from FIG. 2, the MTJ element 1 may have a so-called doubletunnel barrier layer structure. In this case, the MTJ element 1 has atleast ferromagnetic layer 11, insulating layer 12, ferromagnetic layer13, insulating layer 15 and ferromagnetic layer 16, which aresuccessively stacked. The ferromagnetic layer 16 has the fixedmagnetization direction, and functions as a reference layer. This isachieved by providing an anti-ferromagnetic layer 17 above theferromagnetic layer 16. The magnetization direction of the ferromagneticlayer 11 is parallel with that of the ferromagnetic layer 16. Of course,the ferromagnetic layer 16 may have a stacked structure comprisingseveral sub-layers.

It is not essential that these layers 11 to 17 forming the MTJ element 1have all the same shape. In other words, at least the ferromagneticlayer 13 having a variable magnetization has the planar shape shown inFIG. 1.

As illustrated in FIG. 1, the planar shape of the MTJ element 1 iscomposed of a rectangular part 1 a and two projected parts 1 b 1 and 1 b2.

The rectangular part has a rectangular shape as the most typicalexample; in this case, it may have a substantially rectangular shape. Inother words, a square shape is given having the following two longer andshorter sides. Specifically, two longer sides (first and second longersides L1, L2) mutually face each other. Two shorter sides (first andsecond shorter sides S1, S2) mutually face each other and shorter thanthe longer side. Two longer sides and two shorter sides have no need tobe parallel with each other so long as they are arranged along thesimilar direction.

Individual sides of the rectangular part 1 a have no need to be fullystraight line, and part thereof may comprise a curved line. In otherwords, a certain side may include a curved portion as long as the sideis substantially directed along one direction as a whole. The cornerportion of the rectangular part 1 a may be any of right angle, acuteangle and obtuse angle, and further, may be rounded.

Two projected parts (first and second projected parts 1 b 1 and 1 b 2)project along a direction crossing the longitudinal direction of therectangular part 1 a. One projected part (e.g., projected part 1 b 1) isconnected to one longer side (e.g., L1). The other projected part (e.g.,projected part 1 b 2) is connected to the other longer side (e.g., L2).These projected parts 1 b 1 and 1 b 2 are substantially trapezoid. Thewidth of each trapezoid becomes gradually wide from the side far fromthe rectangular part 1 a toward there.

Moreover, one projected part (e.g., projected part 1 b 1) is positionedoff the center of the longitudinal direction of the rectangular part 1 atoward one shorter side (e.g., shorter side S1). The other projectedpart (e.g., projected part b2) is positioned off the center of thelongitudinal direction of the rectangular part 1 a toward the othershorter side (e.g., shorter side S2). Two projected parts 1 b 1 and 1 b2 are positioned between the corner of the rectangular part and thecenter thereof, and not positioned at the corner.

Some corner of the rectangular shape may be rounded on purpose. Therounded corner is one of two corners formed by one longer side and twoshorter sides, that is, a corner on the side where the projected partconnected with the longer side shifts from the center point of therectangular part 1 a. In other words, the rounded corner is two corners,that is, a corner C1 formed by the longer side L1 and the shorter sideS1 and a corner C2 formed by the longer side L2 and the shorter side S2.When remaining two corners, that is, a corner C3 formed by the longerside L1 and the shorter side S2 and a corner C4 formed by the longerside L2 and the shorter side S1 are actually formed to be intentionallyor unintentionally rounded, the curvature of radius of the corners C1and C2 is larger than that of the corners C3 and C4.

The MTJ element 1 has the planer shape described above; therefore, theplanar shape of FIG. 1 is point symmetry with respect to the centerpoint, and non-line symmetry with respect to the longitudinal straightline passing through the center point.

The dimension of each part of the MTJ element 1 having the foregoingshape is as follows. The maximum width is preferably smaller than about1 μm. The maximum width means the center width of the MTJ element 1,that is, a length of two shorter sides of the rectangular part 1 a. Thelength (in the longitudinal direction) is preferably 1 to 10 times asmuch as the maximum width. The thickness of individual ferromagneticlayers 11, 13 and 16 forming the MTJ element 1 is less than 20 nm,preferably, 10 nm. In order to achieve high integration of the MTJelement 1, the element (device) size is preferably smaller.

Magnetic materials such as Fe, Co, Ni, these stacked films and alloysmay be used as a material for ferromagnetic layers 11, 13 and 16.Ferromagnetic layers 11, 13 and 16 may be a stacked film including alayer consisting of nonmagnetic metal such as Cu, Au, Ru and Al. In thefollowing simulation, Ni₈Fe is used as a ferromagnetic material.

Note that FIG. 1 schematically shows the magnetization direction of eachposition (each magnetic domain) shown by an arrow under no externalmagnetic field applied (hereinafter, referred to as zero magnetic filedstate). In FIG. 1, each large arrow shows a rough magnetizationdirection on each position where the arrow is shown and itssurroundings. As seen from FIG. 1, the magnetization of each magneticdomain points to the same direction (right direction) in the most partof the rectangular part 1 a including the middle portion of the MTJelement 1. In projected parts 1 b 1 and 1 b 2, the magnetization of eachmagnetic domain points to the same direction as the rectangular part 1a. Around the shorter sides S1 and S2 of the rectangular part 1 a, themagnetization points to the same direction (from upper left toward lowerright) along each edge of the corners C1 and C2 having a large curvatureof radius.

The magnetic filed pattern is formed in the zero magnetic field state asdescribed above. For this reason, a change of the magnetic filed patternis different between when a magnetic filed is applied to the MTJ element1 in one direction only and when the magnetic field is applied to theMTJ element 1 in two directions. FIG. 4 shows a magnetization state whenmagnetic field is applied to the MTJ element 1 of FIG. 1 in onedirection only. More specifically, FIG. 4 shows a state thatmagnetization is applied in the direction from the right toward the leftin FIG. 4 along the easy magnetization axis (hereinafter, referredsimply to as easy axis) of the MJT element 1. In other words, this statecorresponds to a semi-select memory cell state in a magnetic memorycomprising MTJ elements arrayed like a matrix, as described later. Theeasy axis (direction of the easy magnetization) is defined as thedirection in which the magnetic moment of the ferromagnet is the easiestto point.

As seen from FIG. 4, a ¾ circular magnetic field pattern is formed atindividual upper left and lower right positions in the MTJ element 1around each base of projected parts 1 b 1 and 1 b 2. This is adevelopment of a so-called C-type magnetic domain. Two C-type magneticdomains circulating in the direction reverse to each other are formed.In the middle portion of the MTJ element 1, that is, portion betweenprojected parts 1 b 1 and 1 b 2, magnetization points from the bottom tothe top in FIG. 4. Thus, the magnetization of the MTJ element 1 is hardto be switched. In other words, the magnetic field of the MTJ element 1is hard to be switched in the semi-select state.

On the other hand, in the selected MTJ element, a magnetic field isapplied before it is applied along the easy axis direction.Specifically, a magnetic field is applied in the direction along thehard magnetization axis (hereinafter, referred simply to as hard axis)of the MTJ element 1, that is, the direction from top to bottom in FIG.4. Thus, magnetization points from the top to the bottom in FIG. 4 inthe potion between projected parts 1 b 1 and 1 b 2 by the magnetic fieldapplied to the hard axis direction. As a result, the magnetization ofthe MTJ element 1 is easier to be switched by the magnetic field alongthe easy axis direction then FIG. 1. Therefore, the magnetization of theselected MTJ element 1 is easily switched. The hard axis (direction ofthe hard magnetization) is defined as the direction in which themagnetic moment of the ferromagnet is the hardest to point.

The following is an explanation about simulation results relevant to theMTJ element having the shape shown in FIG. 1. FIG. 5 is a graph showinga magnetization curve obtained by simulation relevant to the MTJ elementhaving the shape shown in FIG. 1.

The dimension of the MTJ element used for this simulation is as follows.The length is 0.72 μm. The width is 0.24 μm near the shorter side of therectangular part. The center width of the MTJ element 1, that is, themaximum width is 0.36 μm. The thickness of individual ferromagneticlayers 11, 13 and 16 is 3 nm. This embodiment is not limited to theforegoing dimensions and various different values may be properly set solong as the MTJ element has the shape having the foregoing features.

In FIG. 5, the broken line shows a change of magnetization of the MTJelement 1 (i.e., recording layer 13) when changing a magnetic filedapplied in the easy axis (longitudinal direction of the MTJ element 1 ofFIG. 1) with no magnetic field applied in the hard axis (i.e., directionperpendicular to the longitudinal direction of FIG. 1) of the MTJelement 1. In other words, the broken line shows the relationshipbetween magnetization of the recording layer 13 of a semi-selected MTJelement and the magnetic field of the easy axis in a magnetic memoryhaving MTJ elements 1 of FIG. 1 arrayed like a matrix.

On the other hand, the solid line in FIG. 5 shows a change ofmagnetization of the MTJ element 1 (i.e., recording layer 13) whenchanging a magnetic filed applied in the easy axis direction with amagnetic field of 40 Oe applied in the hard axis of the MTJ element 1.In other words, the solid line shows the relationship betweenmagnetization of the recording layer 13 of a selected MTJ element andthe magnetic field of the easy axis in a magnetic memory having MTJelements 1 of FIG. 1 arrayed like a matrix.

As seen from FIG. 5, a change of magnetization in accordance with achange of the magnetic field is different between the semi-selected MTJelement and selected MTJ element.

As seen from FIG. 5, the coercivity is about 150 Oe in the easy axis ofthe semi-selected MTJ element. In addition, the magnetization magnitudegradually reduces with an increase of the magnetic field in thesemi-selected MTJ element jumps up when reaching a certain value, andthereafter, is switched after further reduces.

On the other hand, in the selected MTJ element, jump-up, which wouldoccur in the semi-selected MTJ element, does not occur. Moreover, themagnetization slightly increases from zero, and then, is switched whenreaching about 40 Oe. In other words, magnetization switching is sharplymade in the selected MTJ element. Therefore, no intermediatemagnetization state other than “1” an “0” is given. This phenomenonimplies that micro magnetic domains are not generated in the complicatedform in a magnetization switching process.

According to the shape of the MTJ element of this embodiment, spinswitching progress differently between the semi-selected state and theselected state. Moreover, the difference of switching magnetic filed islarge between the semi-selected state and the selected state. Thesefeatures mean that the semi-selected MTJ element 1 has a low possibilityof being unexpectedly switched; in other words, the MTJ element 1 ishard to be disturbed.

In general, a gap or disturbed portion exists in the magnetizationdirection of the ferromagnetic material. When a ratio of residualmagnetization to saturation magnetization becomes less than 1, tunnelmagnetic resistance in the MTJ element using the ferromagnetic materialis lower than one using the ferromagnetic material with no gap ordisturbed portion. On the contrary, according to this embodiment,ferromagnetic layers 11, 13 and 16 have each the same shape includinginsulating layers 12 and 15; therefore, this means that these layers 11,13 and 16 have the same magnetic domain structure. As a result, there isno decrease of the tunnel magnetic resistance in the magnetizationdirection even when the ratio is less than 1.

FIG. 6 shows an asteroid curve (solid line) obtained by simulationrelevant to the MTJ element 1 having the shape of FIG. 1 and using theforegoing dimension and material. In FIG. 6, there is shown an asteroidcurve (broken line) calculated using a MTJ element having a so-calledtrack shape (L-letter shape), for comparison. FIG. 6 is obtained byplotting values standardized with the coercivity of easy axis.

As seen from FIG. 6, in the MTJ element having the track shape, theasteroid curve does not largely dent toward the origin. Thus, theswitching magnetic field (i.e., magnetic field of points positionedoutside the asteroid curve) is large, and also, large with respect tothe coercivity Hc. For this reason, one magnetic filed (e.g., Hx) whichis to be applied in a semi-selected state and large enough to set theMTJ element to a selected state has to be large. The larger the magneticfiled to be applied in a semi-selected state is, the more themagnetization of the recording layer is easy to be switched by onlymagnetization (e.g., Hx) to set a semi-selected state. In other words,thermal stability is low in the semi-selected state.

On the contrary, in the MTJ element 1 having the shape of thisembodiment, the asteroid curve largely dents toward the origin, ascompared with the track shape. Thus, the switching magnetic field issmall, and also, small with respect to the coercivity Hc. Therefore, onemagnetic filed (e.g., Hx) which is to be applied in a semi-selectedstate and large enough to set the MTJ element to a selected state can besmall. As a result, the magnetization of the recording layer is hard tobe switched by only magnetization (e.g., Hx) to set a semi-selectedstate. In other words, thermal stability is high in the semi-selectedstate.

In the MTJ element 1 of this embodiment, a ratio of the residualmagnetization of easy axis direction to saturation magnetization is0.92. This ratio is almost the same as the MTJ element having arectangular shape. This is because edge domains exist in the MTJ element1 of this embodiment.

The MTJ element 1 of the embodiment may have additionally the followingshape features. FIG. 7 is a top plan view showing another MTJ elementaccording to one embodiment of the present invention. As illustrated inFIG. 7, a longer side L1 has a dent D next to the projected part 1 b 1at the side opposite to the shifted direction of the projected part 1 b1 (i.e., corner C3 side). Likewise, a longer side L2 has a dent D nextto the projected part 1 b 2 at the side opposite to the shifteddirection of the projected part 1 b 2 (i.e., corner C4 side).

In the shape shown in FIG. 1, the MTJ element 1 is point symmetry withrespect to the center point, and non-line symmetry with respect to thelongitudinal direction passing the center point.

In FIG. 7, there is schematically shown a magnetization direction on ateach position in a zero magnetic field using an arrow, like FIG. 1. Asshown in FIG. 1, the MTJ element 1 has the foregoing shape; therefore,each magnetic domain points to the following direction. Specifically,the magnetization of each magnetic domain points to the same directionin the most part of the rectangular part 1 a including the middleportion of the MTJ element 1, like the case of FIG. 1. In projectedparts 1 b 1 and 1 b 2, the magnetization of each magnetic domain pointsto the same direction (right direction) as the rectangular part. Inshorter sides S1 and S2 of the rectangular part 1 a, each magnetizationpoints to the same direction (from upper left toward lower right) alongcorners C1 and C2 having a large curvature of radius.

FIG. 8 shows a state of magnetization when a magnetic filed is appliedto the MTJ element of FIG. 7 in one direction only, like the case ofFIG. 4. As depicted in FIG. 8, right and left sides of the MTJ element 1in the longitudinal direction is formed with two C-type magneticdomains, like the case of FIG. 4. In these two C-type magnetic domains,their magnetizations point to the direction opposite to each otheraround the dent D. Therefore, a magnetic filed of the MTJ element 1 ishard to be switched in a semi-selected state.

The selected MTJ element 1 takes the same magnetization pattern as thecase of FIG. 1.

FIG. 9 is a graph showing a magnetization curve obtained by simulationrelevant to the MTJ element having the shape shown in FIG. 7. Thedimension of the MTJ element used for this simulation is as follows. Thelength is 0.76 μm. The width of the edge portion is 0.28 μm. The centerwidth of the MTJ element 1, that is, the maximum width is 0.38 μm. Thethickness of individual ferromagnetic layers 11, 13 and 16 is 3 nm.

In FIG. 9, the broken line shows a change of magnetization of the MTJelement 1 (i.e., recording layer 13) when changing a magnetic filedapplied in the easy axis of the MTJ element 1 with no magnetic fieldapplied in the hard axis of the MTJ element 1, like the case of FIG. 5.In other words, the broken line shows the relationship betweenmagnetization of the recording layer 13 of the semi-selected MTJ element1 and the magnetic field of the easy axis.

On the other hand, the solid line in FIG. 5 shows a change ofmagnetization of the MTJ element 1 (i.e., recording layer 13) whenchanging a magnetic filed applied in the easy axis with a magnetic fieldof 40 Oe applied in the hard axis of the MTJ element 1 like the case ofFIG. 5. In other words, the solid line shows the relationship betweenmagnetization of the recording layer 13 of the selected MTJ element 1and the magnetic field of the easy axis.

As seen from FIG. 9, magnetization switching is sharply made in theselected state MTJ element 1, like the case of FIG. 5. On the otherhand, the magnetization is hard to be switched in the semi-selected MTJelement 1. Moreover, the difference of switching magnetic field is largebetween the semi-selected state and the selected state. This means thatthe MTJ element 1 has high thermal stability, and is hard to bedisturbed, like the case of FIG. 5.

In FIG. 9, switching magnetic field is smaller than the case of FIG. 5.Therefore, a write current required for switching the magnetization ofthe MTJ element having the shape of FIG. 7 is smaller than that ofFIG. 1. As seen from FIG. 9, the residual magnetization state is high,that is, 0.927.

The following is an explanation about the method of manufacturing theMTJ element according to the embodiment. In general, the MTJ element isformed via the following processes. First a stacked film comprisingseveral films forming the MTJ element is formed. Then, a mask materialconsisting of resist is coated on the stacked layer, and thereafter, apattern is transferred into the mask material using light, electron beamor X rays. The mask material is developed, and thereby, a pattern isformed in the mask material. Ion milling or etching is carried out usingthe mask material as a mask, and thereby, the stacked film is patternedinto a shape corresponding to the pattern of the mask material.Thereafter, the mask material is removed.

If a relatively large size, for example, micron-order MTJ element ismanufactured, the following process may be carried out. First, a stackedfilm is formed using sputtering, and thereafter, a hard mask such assilicon oxide and silicon nitride is formed on the stacked film. Then,the hard mask is patterned using reactive ion etching (RIE). Thereafter,the stacked film is subjected to ion milling using the hard mask, andthereby, a MTJ element is formed.

Moreover, if a smaller, for example, sub-micron size MTJ element havinga range from 0.1 to 2 or 3 0.1 μm is manufactured, photo-lithography maybe used. In this case, a hard mask having a shape pattern of MTJ elementis prepared, and the stacked film is patterned using the hard mask.

If a smaller size, for example, MTJ element having 0.5 μm or less ismanufactured, electron beam exposure is used. However, in this case, theelement itself is very small; for this reason, a shape portion forwidening edge domain becomes smaller in the MTJ element of thisembodiment. As a result, it is very difficult to manufacture theforegoing MTJ element. In order to manufacture the MTJ element 1 of thisembodiment, proximity effect correction by electron beam may be used.

Usually, the proximity effect correction involves correcting proximityeffect in a graphic pattern, which is caused by back scattering ofelectron beam from a substrate, to form a pattern loyal to a desiredpattern. For example, if a rectangular pattern is formed, accumulatedelectron charge near the corner is not high enough; for this reason, thecorner of the rectangle may be rounded. In order to clear the corner,correction point beam may be applied outside the pattern in the vicinityof the vertex if a 0.5 μm or less device (element) to be manufactured,in particular. By doing so, the electron charge is increased, andthereby, a pattern loyal to the desired pattern is obtained.

The proximity effect correction can be used to form a shape of the MTJelement having an edge portion whose width is widened. For example, ifshapes shown in FIG. 1 and FIG. 7 are to be formed, a rectangle is usedas a basic pattern, and correction point beam is applied to two cornersfacing each other. By doing so, a shape with a widened width at an edgeis formed. In this case, the shape of the rectangle is controlled inaddition to correction on the shape of the corner in the followingmanner. Specifically, it is controlled by increasing the applied charge,or properly controlling an applied position of the correction pointbeam, or using both of the former and the latter. By doing so, the shapeof the MTJ element 1 according to this embodiment is realized. Theforegoing correction point beam is applied to several points, andthereby, it is possible to form a projected part, and to give aroundness of the corner of a rectangular part.

The MTJ element 1 according to this embodiment is applicable to amagnetic random access memory (MRAM). In general, a random access memoryhas a need to have a small size and large capacity. Thus, it ispreferable that line width and area of each memory cell are small. TheMTJ element 1 serves to realize a high-speed switching memory cellhaving small switching magnetic field, small write current required forwriting data and low power consumption. Therefore, the MTJ element 1 ispreferably applicable to the memory cells of the MRAM.

The case where the MTJ element of this embodiment is used as a memorycell of MRAM will be explained below with reference to FIG. 10 to FIG.12.

FIG. 10 schematically shows a planer layout of a cell array of a MRAM.As shown in FIG. 10, several bit lines BL for writing and reading andword lines WL (write word line WWL) extend to the direction differentfrom each other. Typically, the word line WL extends along the xdirection and the bit line extends along the y direction perpendicularto the x direction. The intersection of the bit line BL and the wordline WL is provided with a memory cell MC including the MTJ element 1.

The longitudinal direction of the MTJ element 1 extends along the wordline WL. Each bit lines BL is electrically connected with one terminalof several MTJ elements 1 in the same row (or column). Each word line WLis arranged to closely face the other end of several MTJ elements 1 inthe same row (or column). For example, the memory cell MC has aso-called cross point structure or so-called 1Tr+1MTJ structure, asdescribed later.

A write current flowing through the word line WL toward the left alongthe x direction applies a magnetic field Hy to the upper direction alongthe y direction to memory cells through which the foregoing word line WLpasses. The write current flowing through the bit line BL toward theupper direction along the y direction applies a magnetic field Hx to theleft direction along the x direction to memory cells through which theforegoing bit line BL passes.

As described above, application of two magnetic fields to the MTJelement 1 of the memory cell MC which is provided at the intersectionpoint of the bit line BL and the word line WL switches the magnetizationdirection of the recording layer 13 of the MTJ element 1. Thus,information is written to a target memory cell MC.

Information is read in by applying a voltage to recording layer 13 andreference layer 11 of the selected memory cell MC, and reading aresistance value from a current flowing through it. Information may beread by flowing a constant current through the MTJ element 1 of theselected memory cell to read a voltage between recording layer 13 andreference layer 11.

FIG. 11 shows an application of the MTJ element 1 of the embodiment to amemory cell having a so-called cross point structure. As depicted inFIG. 11, the memory cell MC comprises the MTJ element 1 only. One end ofthe MTJ element 1 is electrically connected to a bit line BL via anelectrode UE. The other end of the MTJ element 1 is electricallyconnected to a word line WL via an electrode BE.

The bit line BL and word line WL are connected to a driver circuit andsink circuit for carrying a current to a direction corresponding towritten data. These sink circuit and driver circuit are connected to adecoder circuit. The decoder circuit controls sink and driver circuitsso that a magnetic filed is applied to the MTJ element in accordancewith an address signal supplied externally and a current is carriedthrough bit line BL and word line WL. The bit line BL is furtherconnected with a read circuit such as sense amplifier.

FIG. 12 shows an application of the MTJ element 1 of the embodiment to amemory cell having a so-called 1Tr+1MTJ structure. As seen from FIG. 12,a memory cell MC is composed of MTJ element 1 (shown in FIG. 2) and MOS(Metal Oxide Semiconductor) transistor TR used as a read switchingelement.

One end of the MTJ element 1 is electrically connected to a bit line BLvia an upper electrode UE. The other end of the MTJ element 1 iselectrically connected to one end of the switching element TR via bottomelectrode BE, conductive layer and contact plug CP. The other end of theswitching element is grounded.

A diode may be used as the switching element TR in place of the MOStransistor.

The word line (write word line) WL is used to carry a current in anwrite operation, and positioned below the conductive layer IC and apartfrom it. The word line WL is connected to driver circuit and sinkcircuit. These sink circuit and driver circuit are connected to adecoder. The bit line BL is connected to driver circuit, sink circuitand read circuit such as sense amplifier. The switching element TR issupplied with a signal for selecting a predetermined memory cell MC in aread operation via a read word line (not shown).

According to one embodiment of the present invention, the MTJ element 1is composed of substantially rectangular part and two projected parts,which face each other and positioned off the center of the rectangularpart along the longitudinal direction thereof. The foregoing shape canpreferably controls the magnetic structure, in particular, edge domainof the ferromagnetic layer (i.e., ferromagnetic layers 11, 13, 16) ofthe MTJ element 1. When the MTJ element 1 is used as a memory cell of anMRAM, the magnetic structure of the ferromagnetic layer is differentbetween a semi-selected state and a selected state. Thus, large writemargin is secured as compared with the conventional case, and the MTJelement 1 having thermal stability is realized.

Moreover, the thermal stability is high; therefore, even if the MTJelement 1 is made into a small size, a write error is hard to occurresulting from thermal fluctuation. Reducing MTJ element 1 can improvethe integration level of a memory cell when the MTJ element 1 is appliedas a memory cell of a MRAM. Small sized MTJ element 1 with smallswitching magnetic field can reduce power consumption in the MRAM.

This embodiment does not employ the configuration of controlling edgedomains by applying a bias magnetic filed to the MTJ element 1.Therefore, no additional structure for applying the bias magnetic filedis required.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A magnetoresistive effect element comprising: a nonmagnetic layerhaving a first surface and a second surface which face each other; areference layer provided on the first surface and having a fixedmagnetization direction; and a magnetization variable layer provided onthe second surface, having variable magnetization direction, and havinga planer shape including a rectangular part, a first projected part, anda second projected part, the rectangular part having a first longer sideand a second longer side which face each other and a first shorter sideand a second shorter side which face each other, the first projectedpart projecting from the first longer side at a position shifted fromthe center toward the first shorter side, the second projected partprojecting from the second longer side at a position shifted from thecenter toward the second shorter side.
 2. The element according to claim1, wherein the planer shape is a point symmetry with respect to thecenter, and is non-line symmetry with respect to a longitudinallystraight line passing through the center.
 3. The element according toclaim 1, further comprising: a first corner made by the first longerside and the first shorter side, a second corner made by the secondlonger side and the second shorter side, a third corner made by thefirst longer side and the second shorter side, the third corner beingsmaller than the first and second corners, and a fourth corner made bythe second longer side and the first shorter side, the fourth cornerbeing smaller than the first and second corners.
 4. The elementaccording to claim 3, wherein the first to fourth corners comprise acurved line.
 5. The element according to claim 3, wherein the firstlonger side has a dent formed between the first projected part and thethird corner, and the second longer side has a dent formed between thesecond projected part and the fourth corner.
 6. The element according toclaim 1, wherein the first corner made by the first longer side and thefirst shorter side, the second corner made by the second longer side andthe second shorter side, the third corner made by the first longer sideand the second shorter side and the fourth corner made by the secondlonger side and the first shorter side comprise curved lines, acurvature of radius of the first corner is larger than that of the thirdand fourth corners, and a curvature of radius of the second corner islarger than that of the third and fourth corners.
 7. A magnetoresistiveeffect element comprising: a first nonmagnetic layer having a firstsurface and a second surface which face each other; a first referencelayer provided on the first surface and having a fixed magnetizationdirection; a second nonmagnetic layer having a third surface and afourth surface which face each other; a second reference layer providedon the third surface and having a fixed magnetization direction; and amagnetization variable layer interposed between the second and fourthsurfaces, having variable magnetization direction, and having a planershape including a rectangular part, a first projected part, and a secondprojected part, the rectangular part having a first longer side and asecond longer side which face each other and a first shorter side and asecond shorter side which face each other, the first projected partprojecting from the first longer side at a position shifted from thecenter toward the first shorter side, the second projected partprojecting from the second longer side at a position shifted from thecenter toward the second shorter side.
 8. The element according to claim7, wherein the planer shape is a point symmetry with respect to thecenter, and is non-line symmetry with respect to a longitudinallystraight line passing through the center.
 9. The element according toclaim 7, further comprising: a first corner made by the first longerside and the first shorter side, a second corner made by the secondlonger side and the second shorter side, a third corner made by thefirst longer side and the second shorter side, the third corner beingsmaller than the first and second corners, and a fourth corner made bythe second longer side and the first shorter side, the fourth cornerbeing smaller than the first and second corners.
 10. The elementaccording to claim 9, wherein the first to fourth corners comprise acurved line.
 11. The element according to claim 9, wherein the firstlonger side has a dent formed between the first projected part and thethird corner, and the second longer side has a dent formed between thesecond projected part and the fourth corner.
 12. The element accordingto claim 7, wherein the first corner made by the first longer side andthe first shorter side, the second corner made by the second longer sideand the second shorter side, the third corner made by the first longerside and the second shorter side and the fourth corner made by thesecond longer side and the first shorter side comprise curved lines, acurvature of radius of the first corner is larger than that of the thirdand fourth corners, and a curvature of radius of the second corner islarger than that of the third and fourth corners.
 13. A magnetic memorycomprising: a memory cell array having a plurality of magnetoresistiveeffect elements described in claim 1 as memory elements, the pluralityof the magnetoresistive effect elements being arrayed in a matrix; afirst wiring electrically connected to one end of each of the pluralityof the magnetoresistive effect elements belonging to a common row; asecond wiring electrically connected to another end of each of theplurality of magnetoresistive effect elements belonging to a commoncolumn; and a control circuit connected to the first and second wiringsand carrying a current to the first and second wirings to write and readinformation into and from a targeted one of the plurality ofmagnetoresistive effect elements.
 14. A magnetic memory comprising: amemory cell array having a plurality of magnetoresistive effect elementsdescribed in claim 7 as memory elements, the plurality of themagnetoresistive effect elements being arrayed in a matrix; a firstwiring electrically connected to one end of each of the plurality of themagnetoresistive effect elements belonging to a common row; a secondwiring electrically connected to another end of each of the plurality ofmagnetoresistive effect elements belonging to a common column; and acontrol circuit connected to the first and second wirings and carrying acurrent to the first and second wirings to write and read informationinto and from a targeted one of the plurality of magnetoresistive effectelement.